Styrene monomer is the monomer from which the polymer polystyrene and co-polymers of polystyrene, such as high impact polystyrene and acrylonitrile butadiene styrene, are produced. Polymers of styrene are common and valuable plastics that can be used in the production of items from electronics casing to toys to disposable plates and cups. The chemical formula of styrene monomer is C6H5C2H3, and its structure consists of a benzene ring with an attached ethylene group. Styrene is generally produced via the dehydrogenation of ethylbenzene. Ethylbenzene has the chemical formula of C6H5C2H5, and its structure consists of a benzene ring with an attached ethyl group.
Ethylbenzene dehydrogenation takes place in a dehydrogenation reactor system, which comprises a series of dehydrogenation reaction chambers and downstream processing equipment. Superheated steam and ethylbenzene enter the reaction chambers, such as in a steam to hydrocarbon molar ratio of at least 4:1, where a dehydrogenation catalyst catalyzes the conversion of ethylbenzene to styrene. The reaction is generally run at a temperature of around 300° C. to 650° C. and atmospheric to sub-atmospheric pressure, such as around 5 to 20 psia. The mechanism for the dehydrogenation reaction involves the loss of two hydrogen atoms from the ethyl group to form a carbon-carbon double bond. Thus, the chemicals exiting the series of reaction chambers generally comprise styrene, hydrogen gas, and steam, as well as unreacted ethylbenzene and other compounds, which can be referred to as styrene offgas.
From the reaction chambers, the products are cooled and separated into offgas (also known as vent gas), condensate, and organic compounds such as aromatics. The styrene offgas is generally fed to a compressor, and then cooled and condensed to obtain hydrogen-rich gas, which can be used for a variety of purposes, such as providing a fuel source for the superheating of the steam used in the reactor system.
The offgas compressor provides the vacuum that allows the dehydrogenation reaction to take place at sub-atmospheric or low atmospheric pressures. The use of lower pressure increases the duty of the vent gas compressor, due to the increased suction volume as a result of pulling in greater amounts of water and organics/aromatics vapors along with the hydrogen gas. Styrene offgas can contain hydrogen, water vapor, methane, ethylene, carbon monoxide and carbon dioxide, as well as aromatics such as ethylbenzene vapor and toluene, benzene, and styrene vapor. Particularly problematic is the presence of styrene, which readily polymerizes upon exposure to high temperatures or long residence times, such as those encountered in the offgas process stream.
Thus, a need exists for an economic solution for the removal of organic compounds, water vapor and other detrimental compounds from a dehydrogenation offgas stream, such as a styrene offgas stream.